In-band and out-of-band signal detection for automatic gain calibration systems

ABSTRACT

An embodiment of the present invention provides an automatic gain control system for a wireless receiver that quickly differentiates desired in-band signals from high power out-of-band signals that overlap into the target band. The system measures power before and after passing a received signal through a pair of finite impulse response filters that largely restrict the signal&#39;s power to that which is in-band. By comparing the in-band energy of the received signal after filtering to the total signal energy prior to filtering, it is possible to determine whether a new in-band signal has arrived. The presence of this new in-band signal is then verified by a multi-threshold comparison of the normalized self-correlation to verify the presence of a new, desired in-band signal.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to communication systems. Moreparticularly, the invention is directed to receivers in wirelesscommunication systems. Even more particularly, the present invention isdirected to automatic gain control systems for such wirelesscommunication system receivers.

[0003] 2. Background of the Related Art

[0004] The use of receivers in wireless systems such as radio andcellular communication systems is well-known in the art. FIG. 1 shows atypical superheterodyne receiver design 10. Here, a radio frequency (RF)signal is received on antenna 15 and provided to RF amplifier 20. The RFsignal is amplified by the RF amplifier 20 and in mixer 25 mixed with asignal from a local oscillator 30. This produces an intermediatefrequency (IF) signal that is amplified in an IF amplifier 35 andfiltered in a bandpass filter 40. The filtered IF signal is againamplified by an IF amplifier 45 and mixed in a product detector 50 witha signal from a beat frequency oscillator 55. The result is a signalthat is amplified by a baseband amplifier 60 and digitized for furtherprocessing by an analog-to-digital (A/D) converter 65.

[0005] In such receivers, less amplifier gain is needed for strongsignals, and it is important that a very strong signal not be amplifiedto the point that when amplified it distorts received informationsignals, overloads system components and possibly damages thecomponents. For this reason, receivers typically have some sort ofautomatic gain control (AGC) system which controls one or more of thesystem amplifiers 20, 35, 45 and 60 to maintain the amplified signalswithin certain ranges (this control may be, e.g., through a bias appliedto the amplifiers). In FIG. 1, the AGC unit 70 receives an IF inputoutput by the IF amplifier 45 and uses it to generate bias signalscontrolling the RF amplifier 20 and the IF amplifiers 35 and 45.

SUMMARY OF THE INVENTION

[0006] An embodiment of the present invention provides an automatic gaincontrol system for a wireless receiver that quickly differentiatesdesired in-band signals from high power out-of-band signals that overlapinto the target band. The system measures power before and after passinga received signal through a number of filters that largely restrict thesignal's power to that which is in-band. By comparing the in-band energyof the received signal after filtering to the total signal energy priorto filtering, it is possible to determine whether a new in-band signalhas arrived. The presence of this new in-band signal is then verified bya multi-threshold comparison of the normalized self-correlation toverify the presence of a new, desired in-band signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] These and other aspects of an embodiment of the present inventionare better understood by reading the following detailed description ofthe preferred embodiment, taken in conjunction with the accompanyingdrawings, in which:

[0008]FIG. 1 shows the structure of a communications receiver of theprior art;

[0009]FIG. 2 shows the structure of a communications receiver of anembodiment of the present invention;

[0010]FIG. 3 shows the structure of an automatic gain control mechanismin the embodiment of FIG. 2;

[0011]FIGS. 4 and 5 show desired characteristics of a gain-controlledsignal;

[0012]FIG. 6 shows characteristics of A/D converter saturation in theembodiment; and

[0013]FIGS. 7A, 7B, 8A and 8B show characteristics of in-band andout-of-band signals.

DETAILED DESCRIPTION OF PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

[0014] The basic structure of a receiver of an embodiment of the presentinvention is shown in FIG. 2. Here, a wideband antenna 115 receives aradio frequency (RF) RF signal and provides it to an RF amplifier 120,and a particular channel or signal within the band is preferablyselected by varying the local oscillators 130 and 180. In theembodiment, the RF signal preferably conforms to the IEEE 802.11astandard, has a frequency in the 5 GHz band and is quadrature modulatedto carry 6 to 54 Mbps. In this embodiment, the signal can carry up to 54Mbits of data and lies within one of twelve 20 MHz wide slots, eightwithin a 5.151-5.35 GHz band and four within a 5.75-5.85 GHz band. Thesignal in this embodiment is a coded orthogonal frequency divisionmultiplexed (OFDM) signal using 52 subcarriers spaced 312.5 kHz apart.It is understood, however, that while the following detailed descriptionof the present invention is made in the context of an IEEE 802.11asystem, that the inventions described herein have application to manydifferent types of communication systems, and are not limited to systemsoperating within the IEEE 802.11a standard. For example, as describedhereinafter the present invention described operating upon the short andlong training symbols in an IEEE 802.11a system, but it is noted thatthe teachings related thereto can be generalized to any trainingsequence made up of one or more sinusoids. Thus, for example, powermeasurements can be made based upon half of a period of a slowestfrequency sinusoid that exists within a training symbol containing aplurality of sinusoids, with each of the plurality of sinusoids having afrequency that is an integer multiple of the slowest frequency sinusoid.In 802.11a system this translates to half of a short training symbolsequence.

[0015] The amplified RF signal is mixed with a signal from a local RFoscillator 130 supplied to an RF mixer 125 to generate an intermediatefrequency (IF) signal that is fed to an IF amplifier 135. Preferably,the sum of the frequencies of the local RF oscillator 130 and local IFoscillator 180 are in the range 5.15-5.35 and 5.75-5.85 GHz, with theratio of the RF oscillator frequency to the IF oscillator frequencybeing 4:1. In the embodiment, the local oscillators 130 and 180 arepreferably in a floating IF arrangement in which they both are variable,rather than a fixed IF arrangement where, e.g., only the RF localoscillator 130 is variable.

[0016] The amplified IF signals are supplied to an in-phase mixer 175-IPand a quadrature mixer 175-Q, respectively. One of the in-phase mixer175-IP and the quadrature mixer 175-Q is directly driven by a local IFoscillator 180, and the other of the in-phase mixer 175-IP and thequadrature mixer 175-Q is driven by the local IF oscillator signal afterit is phase-shifted by 90° in a phase shifter 185. In this way, in-phase(IP) and quadrature (Q) components of the received RF signal areobtained at the outputs of the in-phase mixer 175-IP and quadraturemixer 175-Q, respectively.

[0017] The mixed IF signals pass through low-pass filters 140-IP and140-Q to select the desired channel and remove spectrally distantcomponents not of interest, and are amplified by two series of basebandamplifiers 145-IP and 145-Q. Though two baseband amplifiers are shown ineach branch, a different number of amplifiers may be used. Almost anydesired baseband gain step arrangement may be developed using basebandamplifiers having appropriately selected programmable gains in aparticular order.

[0018] Preferably, the low-pass filters 140-IP and 140-Q are two-poleelliptical filters having a 3 dB corner at 28 MHz. Moving from theanalog to digital domain, the baseband amplifier outputs are fed to A/Dconverters 190-IP and 190-Q which digitize the in-phase and quadraturecomponent signals, preferably with a frequency of 80 MHz, to aresolution of nine bits, and an input dynamic range of −500 mV to 500mV.

[0019] Preferably, the A/D converters are pipeline A/D converters;however, the invention is not so limited. For example, sigma-delta orother converters may be used in their place.

[0020] An analog channel filter and/or anti-aliasing filter mayadvantageously be placed before the A/D converters 190-IP and 190-Q. Inthe preferred embodiment, the combination of the analog filters performadjacent blocker rejection of 4 dB and an alternate blocker rejection of20 dB. With a worst case of an adjacent blocker 16 dB larger and analternate blocker 32 dB larger, a received blocker at the A/D converterinput can be 12 dB higher than the in-band signal. As is known in theart, an adjacent blocker is an interference signal adjacent to oroverlapping the frequency band of interest, while an alternate blockeris an interference signal farther away from the frequency band ofinterest.

[0021] The digitized I/Q component signals are provided to an automaticgain control (AGC) unit 170 whose operation with respect to the presentinvention will be described in greater detail below. The AGC 170analyzes the I/Q component signals as described in greater detail belowand generates gain control signals based thereon. These gain controlsignals are provided to the amplifiers 120, 135, 145-IP and 145-Q asshown by the dotted line in FIG. 2.

[0022] More specifically, as shown in FIG. 3 the digitized IF signalsfrom the A/D converters 190-IP and 190-Q are passed through leaky bucketfilters 245-IP and 245-Q and finite impulse response (FIR) filters205-IP, 210-IP and 205-Q, 210-Q. The first FIRs 205-IP and 205-Q aredecimation filters that eliminate every other sample from theirrespective streams to reduce the data sampling rate from 80 MHz to 40MHz for a normal 8.5 MHz single-sided bandwidth packet. The second FIRs210-IP and 205-Q are standard low-pass filters which remove any residualadjacent or aliased blockers before sending the data to theself-correlator 225 and a power detector 220. Two power measurements aretaken within the AGC 170—one from the output of the second FIR filter210 by the power detector 220, and another from the output of A/Dconverters 190-IP and 190-Q by another power detector 215. Thesemeasurements are provided to AGC control logic 230 as will be describedin greater detail below.

[0023] Although this embodiment uses digital FIRs, other types offilters, including analog filters, may be used in their place. If thesystem is not oversampled, the filters are preferably analog.

[0024] AGC control logic 230 receives the power measurements from powerdetectors 215 and 220 and uses them to control a gain control generator235 to output analog gain control signals for each of the RF amplifier120, the IF amplifier 135, and individual ones of the basebandamplifiers 145-IP and 145-Q. In the embodiment, the AGC control logic230 provides a control word, ten bits in length in the preferredembodiment, to the gain control generator 235, and the gain controlgenerator 235 generates appropriate control signals for the amplifiers.These gain control signals are fed back to the RF amplifier 120, the IFamplifier 135 and the baseband amplifiers 145-IP and 145-Q to controlthe gain of each as described above.

[0025] The embodiment uses an application-specific integrated circuit toimplement the AGC control logic 230; however, an appropriatelyprogrammed processor, either embedded or discrete, or other appropriatedevice, may be used as well.

[0026] It should be noted that although FIG. 3 shows various componentswithin the AGC 170 to be separate from one another, it is possible thattwo or more units may be integrated into one. For example, the AGCcontrol logic 230 is shown separately from the FIRs 205, 210, powerdetectors 215, 220 and self-correlation unit 225; however, several ofthese may be combined into a single processor appropriately programmedto perform these functions. Further, a programmed processor need not beused and one or more of these components can be implemented in dedicatedhardware.

[0027] The AGC 170 may control a DC offset control unit 240 to provideanalog offset control signals to one or more of the baseband amplifiers145-IP and 145-Q. DC offset control is done to ensure that the analogsignals provided to the amplifiers and A/D converters 190-IP and 190-Qare properly centered and quantized.

[0028] AGC Operation

[0029] In the embodiment, the control logic 230 first checks to see ifthe signal is sufficiently saturating either of the A/D converters 190-IP and 190-Q. If so, a quick drop gain control procedure is executed;if not, a base gain control procedure, also described below, isexecuted.

[0030] Next, the AGC base gain control logic 230 determines whether thereceived signal is within a preferred range as described below. If so,no gain control is needed; otherwise, a gain control procedure describedin greater detail below is executed.

[0031] Then, the AGC system 170 attempts to identify an in-band signalusing strong signal and weak signal detection techniques, as describedin greater detail below. If a signal is found, the detection process iscomplete; if not, the detection process is repeated on the next portionof the signal. Weak signal detection and strong signal detection areindependent and complementary features. As described further herein, forstrong signal detection, it is determined that a signal may exist by thearrival of a stronger signal necessitating a drop in receive gain,whereas for weak signal detection, it is determined that a signal mayexist due to a sudden increase in measured in-band power at leastproportional to the increase in total power at the ACC (while notrequiring a gain change), followed shortly by a self-correlationexceeding thresholds. It is noted that it is preferable to disable weaksignal detection, typically for a few microseconds, if a gain change ismade, since self-correlation will not be valid until the entire viewingwindow for self-correlation is filled with post-gain-change values.Thus, weak-signal detection is used for arriving signals not largeenough relative to blockers or noise to cause gain changes, and strongsignal detection for larger arriving signals. And for strong signaldetection, that a new signal has arrived is determined based uponwhether a coarse gain drop or quickdrop gain results, as describedbelow.

[0032] AGC Base Gain Control For a Coarse Gain Change

[0033] In operation, the AGC 170 must adjust receiver gains so that thereceived signal can properly be quantized by the A/D converter 190. Ifthis signal is too big at the A/D converter input, the signal will bedistorted by saturation. If the signal is too small at the A/D converterinput, the quantization noise of the A/D converter 190 will render thereceived signal-to-noise (S/N) ratio too low for correct detection. Forthis purpose, the AGC control logic 230 digitally controls the analogvariable gain stages mentioned above using the gain control unit 235.Preferably, the embodiment's gain control has a dynamic range of 93 dB-51 dB in the combined RF and IF stages 120 and 135 and 42 dB in thebaseband stage 145.

[0034] The power detector 215 estimates the total digitized power at theA/D converters 190-IP and 190-Q by summing a window of instantaneouspower calculations for half of a preamble short symbol window in an802.11a signal (400 ns) for a total of 16 samples. For example, considera signal coming out of a nine-bit A/D converter 190 with a range of[−256, 255], and measurement of power for this signal over a 16-bitsample window in half a preamble short symbol window. To do this, theAGC control logic 230 calculates the instantaneous power adcpwr1 on theA/D converter output stream adcoutput as $\begin{matrix}{{adcpwr1} = {{\sum\limits_{k = 0}^{15}\quad \left( {{real}\left( {{adcoutput}\quad\lbrack k\rbrack} \right)} \right)^{2}} + \left( {{imag}\left( {{adcoutput}\quad\lbrack k\rbrack} \right)} \right)^{2}}} & (1)\end{matrix}$

[0035] This power measurement is then put into a log table, where itsmaximum value is zero. Thus, for a fully railed output with every valueat −256, the logarithmic table output would be zero. The power of afull-rail sinusoid would be −3 dB; if every sample were 128, the powerwould be −6 dB, etc.

[0036] The AGC control logic 230 uses this total power estimate to keepthe signal in-range at the A/D converters 190-IP and 190-Q. If thesignal power is determined to be out of range (but not saturating theA/D converters 190-IP and 190-Q), a coarse gain change will be made toput the signal back in range. More specifically, if AGC control logic230 detects the total measured power adcpwr1 (in the embodiment, withinthe range −63-0 dB) is greater than the maximum desired A/D convertersignal size, the desired gain value gaintarget, which is a signal sizethat is set large enough so that quantization noise is small enough, butalso small enough that ADC saturation is not an issue, including thesize of the signal and any potential blocker, is reduced in a coursegain drop by the AGC control logic 230 of the equation

gaintarget=gaintarget+(coarsepwr _(—) const−adcpwr1)  (2)

[0037] where coarsepwr_const is an additional gain for coarse gain drop(FIG. 4), e.g., −17 dB. This additional gain loss is used because theincoming signal may be too large to quantize but not large enough totrigger a quick drop as described in greater detail below—for example,if the signal saturates occasionally but not enough to trigger a quickdrop. In such cases, it is useful to drop the gain by more than thegaintarget value indicates, based on power measurements of a saturatedwaveform—a very aggressive drop. Thus, the empirically determinedcoarsepwr_const value is added to increase the gain drop to more quicklyconverge on the desired signal size. The result is used to generateappropriate control signals for the amplifiers via the gain controlgenerator 235.

[0038] If the total measured power adcpwr1 is less than the minimumdesired A/D converter signal size, the desired gain value gaintarget isincreased by the AGC control logic 230 of the equation

gaintarget=gaintarget+(totalsizedesired−adcpwr1)  (3)

[0039] where totalsizedesired is the target A/D converter signal sizeduring coarse gain changes, i.e., the desired size of the A/D converteroutput in the absence of a desired signal (FIG. 5)—about −17 dB in thepreferred embodiment.

[0040] AGC Quick Drop Gain Control

[0041] If the received signal is saturating the A/D converters 190-IPand 190-Q often, a precise power measurement may not be obtained;however, it is certain that the signal is well out of range. Thisinformation can be used to quickly reduce the gain. More specifically, asaturation counter adcsat is established by the AGC control logic 230 tocount the number of saturations of either the I or Q A/D converteroutput samples. A pair of saturation thresholds adcsat_thrh andadcsat_thrl, which can be changed by downloading a different threshold,are used to counter any possible lack of A/D converter range. Thus, asaturation will be detected if

adcoutput≧(adcsat _(—) thrh+192)  (4)

[0042] or if

adcoutput≦(adesat _(—) thrl−256)  (5)

[0043] where adcsat_thrh is a high threshold less than the maximum A/Dconverter output value which designates saturation on the high side ofthe A/D converter output, adcsat_thrl is a low threshold value greaterthan the minimum A/D converter output value which designates saturationon the low side of the A/D converter output (FIG. 6) and the constantvalues are implementation-dependent. adcsat_thrh is set to be slightlyless than the maximum A/D converter output, while adcsat_thrl is set tobe slightly higher than the minimum A/D converter output. This is usefulbecause it allows signals that are close to saturation of, but do notactually saturate, the A/D converter to be classified as saturationsignals for more flexibility. If the number of saturations of the A/Dconverter output samples during a sample window of adcsat_icount cycles(preferably less than or equal to eight, the number of cycles in thequarter-symbol 802.11a measurement window) exceeds a saturationthreshold amount adcsat_thresh a quick gain drop is instructed by theAGC control logic 230, and gaintarget is reduced by a predeterminedamount quick_drop, e.g., a −30 dB change in gain. In the preferredembodiment, the adcsat_thresh is set for at a threshold of 12saturations in an 8-cycle window (with saturations independentlypossible on I and Q ADCs).

[0044] This technique may advantageously be implemented in the followingway. After calibration or any gain change, an AGC settling time occurs.After that adcpwr1, the variable corresponding to the amount of measuredpower, is reset and an acc_count counter, preferably an eight-bitincremental counter cycling continuously during AGC operation, also isreset.

[0045] The following events will happen of the counter acc_count:

[0046] mod(acc_count, 16)=0: reset adcpwr1 accumulator

[0047] mod(acc_count, 16)=1: clear reset on adcpwr1 accumulator

[0048] mod(acc_count, 16)=2: store adcpwr1

[0049] mod(acc_count, 16)≦adcsat_icount and adcsat=1 (asserted whenset_thresh saturations, e.g. eight saturations, have been counted), thesaturation counter has exceeded adcsat_thresh and a quick gain dropshould be executed. As shown, in the preferred embodiment, the adcpwr1values are computed every 16 cycles, and the system looks for adcsat tobe asserted prior to the first 4 bits of the counter registering a valuegreater than adcsat_icount (preferably 8).

[0050] AGC Packet Detection

[0051] Once the received signal is in-range, the AGC control logic 230detects the presence of a desired packet. For this purpose, the AGCcontrol logic 230 determines an in-band power estimation, uses the FIRfilters 205-IP, 210-IP and 205-Q and 210-Q to reduce all adjacent andalternate blockers to 20 dB below the in-band signal power at 802.11aspecified maximum levels, and compares adcpwr1 and firpwr1 as describedhereinafter. This is done to obtain information about whether quantizedsignal energy at the A/D converter 190-IP or 190-Q is in-band orout-of-band—information which helps in finding the desired packets.

[0052] More specifically, consider the signal shown in FIG. 7A.Calculating the power of an A/D converter output as described abovemight determine that it has an overall power of, say, −12 dBr, where dBris a measure of the RMS size of signals below the full rail signal sizedescribed above with reference to Equation (1). Passing through thesecond FIR 210-IP or 210-Q as shown in FIG. 7B, however, the signalloses most of its power and is reduced to a level of about −25 dBr—adecrease of roughly 85%. Since most of the signal's power was blocked bythe bandpass FIR 210-IP or 210-Q, it is presumed to be an out-of-bandsignal.

[0053] Referring to the signal shown in FIG. 8A, this signal too has anoverall power of about −12 BDr as measured at the A/D converter output.Passing through the second FIR 210-IP or 210-Q as shown in FIG. 8B,however, only reduces its power to approximately −15 dBr—a decrease ofonly about 10%. Since most of the signal's power was passed by thebandpass FIR 210-IP or 210-Q, it is presumed to be an in-band signal.

[0054] With this understanding, the in-band power is calculated as thesum of instantaneous power measurements, preferably in a 32 sample, 0.8μs window similar to the overall power calculation adcpwr1 describedabove. firpwr1 is the power based on the lowest of some number ofsamples that is less than the entire number of samples obtained, such as28 out of 32 samples in the 32 sample window in detector 220. It isnoted that the number of samples forfirpwr1 is greater than the numberof samples for adcpwr1 because firpwr1 is being used for fine gaincontrol, where precision is important, whereas adcpwr1 is being used forcoarse gain changes, where a slightly noisy power estimate will do. Itis also noted that while for purposes of this in-band power calculationless than the entire number of samples is preferably used, that otherpost analog to digital converter processing that takes place using suchsamples will typically use all the samples obtained.

[0055] This less than the entire number of samples is used becauseduring periods of interference, e.g., at symbol boundaries of theinterferers, a temporary in-band power spike may occur due tohigh-frequency components of interferers at the symbol transitionbecoming in-band components in the desired band. This will artificiallyshow up as a step in the in-band power. Windowing at the transmitter ofthe interferer, e.g., using a value which is half the previous valueadded to half the subsequent value at the symbol boundary, reduces thissomewhat, as does lowpass filtering, so that the aggregate spectrumpasses the necessary spectral mask. These instantaneous high frequencypeaks, although lowered, can still exist. When an adjacent interferer ispresent, this temporary high frequency component in the interferer isactually in-band for the desired signal, so that the in-band powermeasurement when no desired signal is present can get a quick spike fora few samples, looking like an increase in the in-band power. To combatthis, the lowest 28 of the 32 samples are used so this temporary spikeis nulled out by not counting those values, and thresholds are adjustedaccordingly to compensate for the reduced power measurement due to themissing four samples. Once a signal of interest is present, however, allsamples are preferably used in creating the power measurement to make adetailed measurement. This second power measurement is calledfirpwr_all.Using the power information described above, desired signals can befound in two ways: strong signal detection and weak signal detection.Strong signal detection will be described first.

[0056] Strong Signal Detection

[0057] Any time a coarse gain drop or quick gain drop as described aboveoccurs, a flag strongsignal is set by the AGC control logic 230. Thisflag remains high until the signal is determined to be in range at theA/D converter 190-IP or 190-Q, and the algorithm proceeds to makeafirpwr1 measurement as described above. At this point, flag_relpwr iscalculated as

flag _(—) relpwr=set if (firpwr1>relpwr+adcpwr)  (6)

[0058] (where relpwr is an empirical thresholding variable related tothe absolute digital size of the in-band signal relative to the absolutetotal digital signal at the A/D converter 190-IP or 190-Q), thusattempting to see that most of the computed power is in-band. Ifflag_relpwr is high and strongsignal is high, then a new, very strongin-band signal has appeared. In this way, the embodiment permitsexamination of an oversampled incoming signal having digitizedfrequencies beyond a desired frequency range due to oversampling, anddetermine whether most of its power is in-band before determining that adesired signal has been found.

[0059] Thus, when flag_relpwr is high and strongsignal is high, thesignal_found flag is asserted, a fine gain change is made as describedbelow and the AGC process is completed once the number of consecutivegain changes is equal to or greater than the minimum number of gainchanges deemed to constitute a successful AGC operation, i.e., whenthere have been enough gain changes to ensure a full programmableamplifier ramp-up when the system is turned on.

[0060] Weak Signal Detection

[0061] In weak signal detection, the normalized self-correlation ofshort sequences as defined below is measured to look for anythingin-band with a periodicity of 0.8 μs in the preferred embodiment. Thisis a two-step process performed concurrently with the above-describedstrong signal detection process. First, the system waits for thenormalized self-correlation as measured by the self-correlationprocessor 225 to exceed a first normalized self-correlation magnitudethreshold value m1thres.

[0062] The self-correlation processor 225 preferably measuresself-correlation of 802.11a packets by taking 32 samples in a shorttraining symbol at the beginning of a packet and comparing each of thesamples to a corresponding sample from the preceding short trainingsymbol. More specifically, the self-correlation of an A/D converterstream adcoutput is given by $\begin{matrix}{{self\_ corr} = \frac{\left\lbrack {\sum{{{adcoutput}\lbrack n\rbrack} \cdot {{conj}\left( {{adcoutput}\left\lbrack {n - 32} \right\rbrack} \right)}}} \right\rbrack^{2}}{\sum{{adcoutput}\lbrack n\rbrack}^{2}}} & (7)\end{matrix}$

[0063] where the denominator is a normalization factor. One can see thatthe numerator will be relatively high when x[n] and x[n−32] areidentical and relatively low when, e.g., they are uncorrelated. Thus,this measure can serve as a good indicator of self-correlation.

[0064] Detecting when the self-correlation output exceeds m1thres canthus detect the existence of an incoming packet; however, it would alsodetect interferers, since they can have structures that can alsoself-correlate. For this reason, the embodiment advantageously employsanother test. Once the normalized self-correlation exceeds ml thres, thesystem enters a loop and for m1count_max cycles counts in a variable mltally the number of times the normalized self-correlation exceeds asecond normalized self-correlation magnitude threshold value m2thres,where m2thres is less than or equal to m1thres. If m1tally>m2count_thr,a threshold of the count of normalized self-correlation >m2thres, beforem1count_max (a window length for the self-correlation count) cycles haveelapsed, weak signal detection may be detected.

[0065] As noted above, the windowing technique based on m1count_max isused because both interferers and noise may have a self-correlation thatmomentarily exceeds a threshold, but the chances of this occurringdiminish when windows of samples obtained over consecutive periods oftime are used. For example, a subsequent window will contain many of thesame samples as the previous window, but the previous window will notcontain the most recent sample from the subsequent window, and thesubsequent window will not contain the oldest sample from the previouswindow. Thus, for example, if two 802.11a symbols in adjacent channelsare sent, such that they are separated in frequency by 20 MHz, the last0.8 μs of the first symbol will exactly match the next 0.8 μs guardperiod of the next symbol, creating self-correlation, but this spikewill rapidly fade, in comparison with a preamble where a flat normalizedself-correlation result is expected for the preamble duration.

[0066] Thus, the embodiment provides a way of performing a two-thresholdwindowing process on a self-correlation measurement. One threshold isused to determine that a signal may be present in-band, and the numberof times a second threshold is exceeded in different windows of offsetsamples is counted to further determine if that in-band signal is adesired signal. This is done to combat temporary correlation of thermalnoise as well as to combat self-correlation during the data segment ofan interferer.

[0067] Additionally, for further robustness against thermal noise andinterferers, the embodiment preferably requires that to enable a weaksignal detection result, a potential detected packet must increase thein-band signal power by at least a certain amount and that the increasebe at least proportional to any increase in the total signal power, thesignal power being of at least a certain minimum size. This providesextra sensitivity when a new in-band signal comes in below an interfereror near the noise floor, thus not triggering strong signal detection butworthy of a look for weak signal detection.

[0068] At least three things may stop weak signal detection fromoccurring once m1tally >m1thresh. First, if ycOK=0, weak signaldetection will not occur. ycOK is a decrementing counter that is resetto ycOKmax (in the embodiment, four) to enable weak signal detection ifit is determined that an increase in the in-band signal of a certainsize (flag_firstep) and at least proportional to any increase in thetotal power (flag relstep) with the measuredfirpwr1 of at least acertain minimum size (flag_firpwr), then it is possible that a newin-band signal has come in below an interferer or near the noise floor,thus not triggering strong signal detection but worthy of a look forweak signal detection. To ensure that such recognition occurs within alimited period of time, the above must happen while ycOK>0 if it happensat all. To perform these step calculations, old values offirpwr1 andadcpwr are stored asfirpwr{2-4} and adcpwr{2-4}. Enough values arestored so that if the signal is detected during a programmable amplifierramp, enough difference will exist between the first and lastmeasurements to exceed the given threshold.

[0069] Another reason why weak signal detection might not occur isbecause gc_count is greater than zero. gc_count measures the time sincethe last gain change in short symbol increments, getting decremented bythe AGC control logic 230 for every validfirpwr1 measurement from itsstarting value of three after a gain change. The idea is that after again change, there is a minimum amount of time until a self-correlationis valid.

[0070] Finally, weak signal detection will not occur if the signal hasalready been found with another method, since then there is no need tofind it using weak signal detection.

[0071] AGC Packet Detection—DC Offset Elimination

[0072] The above double threshold arrangement is successful in reducingfalse packet detects on interferers during weak signal detection;however, it is not particularly successful in preventing false detectionwith respect to DC signals, which always self-correlate. There istypically a small DC component at the output of the A/D converter 190,so the embodiment uses a two-tap DC notch filter as a leaky bucketfilter—more specifically, a two-tap IIR filter having a transfer curveof the form $\begin{matrix}{{y\lbrack n\rbrack} = {{\frac{\alpha - 1}{\alpha}{y\left\lbrack {n - 1} \right\rbrack}} + {\frac{1}{\alpha}{x\lbrack n\rbrack}}}} & (8)\end{matrix}$

[0073] where x is the input signal, y is the output signal and α is afilter parameter (in this case, 32)—which uses an estimate of the DClevel provided by the AGC logic control 230 to cancel the DC componentout. The AGC control logic 230 obtains this level from a lookup tablebased on current gain settings.

[0074] AGC Completion Processing

[0075] Once the signal is found via either strong signal detection orweak signal detection, fine gain changes will be made, in the preferredembodiment if consec_gainchanges<min_gainchanges. And in the preferredembodiment, every fine gain change will be made based upon the equation

gain _(—) change=adc _(—) desired _(—) size−firpwr1_(—) all  (9)

[0076] consec_gainchanges begins at zero for strong signal detection andtwo for weak signal detection, since it is meant to be a coarse measureof time spent in the AGC, and it takes approximately two gain changetimes to perform a windowed self-correlation. It is incremented everycoarse and fine gain change. It is reset when no gain change is made andstrong signal detection does not decide that a signal is present. Thisfeature is meant to ensure that a minimum amount of time is spent in theAGC, for either more precise gain or to be sure the gain is set afterthe PA is done ramping.

[0077] The preferred embodiments described above have been presented forpurposes of explanation only, and the present invention should not beconstrued to be so limited. Variations on the present invention willbecome readily apparent to those skilled in the art after reading thisdescription, and the present invention and appended claims are intendedto encompass such variations as well.

What is claimed is:
 1. A method of estimating whether a received strongsignal is an in-band signal based upon power estimates, the methodcomprising the steps of: inputting the received strong signal; measuringthe power of the received strong signal; providing the received strongsignal to a filter section; using the filter section to pass frequencycomponents of the received strong signal within a desired band offrequencies to obtain a filtered signal; measuring the power of thefiltered signal and comparing the power measurement of the receivedstrong signal to the power measurement of the filtered signal to obtainan indication that the received signal is estimated as being in-band. 2.The method of claim 1, wherein measuring the power of the receivedstrong signal and measuring the power of the filtered signal areperformed by taking samples within windows of the received strong signaland the filtered signal, respectively.
 3. The method of claim 2, whereinextreme samples in the measurement of the received strong signal and inthe measurement of the filtered signal are not used in the respectivemeasurements.
 4. The method of claim 3, wherein highest magnitudesamples in the measurement of the received strong signal and in themeasurement of the filtered signal are not used in the respectivemeasurements.
 5. The method of claim 4 wherein prior to the step ofmeasuring the power of the received strong signal there is included thestep of removing DC offset from the input signal.
 6. The method of claim4 wherein the step of measuring the power of the received strong signaluses a different number of signal samples than the step of measuring thepower of the filtered signal.
 7. The method of claim 4 wherein thereceived signal power measurement is made based upon less than areceived whole short training symbol.
 8. The method of claim 4 whereinthe received signal power measurement is made based on half of a periodof a slowest frequency sinusoid that exists within a training symbolcontaining a plurality of sinusoids, with each of the plurality ofsinusoids having a frequency that is an integer multiple of the slowestfrequency sinusoid.
 9. The method of claim 2 wherein prior to the stepof measuring the power of the received strong signal there is includedthe step of removing DC offset from the input signal.
 10. The method ofclaim 2 wherein the step of measuring the power of the received strongsignal uses a different number of signal samples than the step ofmeasuring the power of the filtered signal.
 11. The method of claim 1,wherein comparing the power measurements of the received strong signaland the filtered signal includes determining whether the filtered signalpower measurement is at least a certain portion of the received signalpower measurement.
 12. The method of claim 1 wherein prior to the stepof measuring the power of the received strong signal there is includedthe step of removing DC offset from the input signal.
 13. The method ofclaim 1, further including the step of determining that the receivedstrong signal is a desired signal after the indication has beenprovided.
 14. The method of claim 1 wherein the received signal powermeasurement is performed on less than a whole short training symbol. 15.The method of claim 14 wherein the received signal power measurement isperformed on about one half of a received whole short training symbol.16. A method of estimating whether an input signal is an in-band signal,the method comprising the steps of: inputting the input signal;converting the input signal to a received digital signal using an analogto digital converter; determining whether the received digital signal issaturating the analog to digital converter and if so identifying thedigital signal as a strong signal type; measuring the power of thereceived digital signal; determining whether the power of the receiveddigital signal is greater than a desired level and if so identifying thereceived digital signal as the strong signal type; providing thereceived digital signal to a filter section; using the filter section topass frequency components of the received digital signal within adesired band of frequencies to obtain a filtered signal; measuring thepower of the filtered signal and comparing the power measurement of thereceived digital signal to the power measurement of the filtered digitalsignal to obtain an indication that the received signal is an in-bandsignal type; and determining that the received digital signal is anin-band signal if the received digital signal is indicated as being ofthe in-band type and is identified as being of the strong signal type.17. The method of claim 16 further including the steps of: reducing again provided by an automatic gain control circuit to an automatic gaincontrol amplification section if saturation is determined; and reducinga gain provided by the automatic gain control circuit to the automaticgain control amplification section if the power of the received digitalsignal is greater than the desired level.
 18. The method of claim 16,wherein measuring the power of the received digial digital signal andmeasuring the power of the filtered signal are performed by takingsamples within windows of the received digital signal and the filteredsignal, respectively.
 19. The method of claim 18, wherein extremesamples in the measurement of the received digital signal and in themeasurement of the filtered signal are not used in the respectivemeasurements.
 20. The method of claim 19, wherein highest magnitudesamples in the measurement of the received digital signal and in themeasurement of the filtered signal are not used in the respectivemeasurements.
 21. The method of claim 20 wherein prior to the step ofmeasuring the power of the received digital signal there is included thestep of removing DC offset from the input signal.
 22. The method ofclaim 20 wherein the step of measuring the power of the received digitalsignal uses a different number of signal samples than the step ofmeasuring the power of the filtered signal.
 23. The method of claim 20wherein the received digital signal power measurement is performed onless than a received whole short training symbol.
 24. The method ofclaim 23 wherein the received digital signal power measurement isperformed on about one half of a received whole short training symbol.25. The method of claim 18 wherein prior to the step of measuring thepower of the received digital signal there is included the step ofremoving DC offset from the input signal.
 26. The method of claim 18wherein the step of measuring the power of the received digital signaluses a different number of signal samples than the step of measuring thepower of the filtered signal.
 27. The method of claim 18, whereincomparing the power measurements of the received strong signal and thefiltered signal includes determining whether the filtered signal powermeasurement is at least a certain portion of the received signal powermeasurement.
 28. The method of claim 16 wherein prior to the step ofmeasuring the power of the received strong signal there is included thestep of removing DC offset from the input signal.
 29. The method ofclaim 16 wherein the received signal power measurement is determinedbase upon less than a whole short training symbol.
 30. The method ofclaim 29 wherein the received signal power measurement is determinedbased upon about one half of a received whole short training symbol. 31.The method of claim 16 further including the step of making a fineautomatic gain control adjustment if it was determined that an in-bandsignal exists.
 32. A method of determining that a received digitalsignal of the strong signal type exists within an input signal includingthe steps of: converting the input signal to a received digital signalusing an analog to digital converter; measuring the power of thereceived digital signal; and determining whether the power of thereceived digital signal is greater than a desired level and if soidentifying the received digital signal as the strong signal type; 33.The method of claim 32 further including the step of reducing a gainprovided by an automatic gain control circuit to an automatic gaincontrol amplification section if the power of the received digitalsignal is greater than the desired level.
 34. The method of claim 33wherein the step of reducing the gain reduces the gain by an amount thatis based upon the determined power and a predetermined gain targetvalue.
 35. The method of claim 32 further including the steps of:determining whether the received digital signal saturates the analog todigital converter and if so identifying the digital signal as the strongsignal type.
 36. The method of claim 35 further including the steps of:determining whether the received digital signal saturates the analog todigital converter; and reducing a gain provided by an automatic gaincontrol circuit to an automatic gain control amplification section ifthe received digital signal is determined to saturate the analog todigital converter.
 37. The method of claim 36 wherein the step ofreducing the gain reduces the gain a predetermined amount.
 38. Themethod of claim 35 wherein the step of determining whether the receiveddigital signal saturates the analog to digital converter includes thesteps of: counting a number of times that a plurality of samples of thereceived digital signal exceeded the predetermined saturation threshold;and comparing the number of times with a predetermined saturation countthreshold, such that when the predetermined saturation threshold isexceeded, the received digital signal is determined to saturate theanalog to digital converter.
 39. A system for detecting in-band signals,the system comprising: a filter section for filtering a received digitalsignal to pass frequency components of the received digital signalwithin a desired band of frequencies to obtain a filtered signal; apower detector for measuring power of the filtered signal and formeasuring power of the received digital signal; and control logic for,based on the measured power of the received digital signal and thefiltered signal, determining whether the received signal is an in-bandsignal.
 40. The system of claim 39, wherein the filter section includesa finite impulse response filter.
 41. The system of claim 39, whereinthe filter section includes a decimation filter.
 42. The system of claim39, wherein the filter section includes a low-pass filter.
 43. Thesystem of claim 36, wherein the power detector includes separate powerdetectors for measuring the received digital signal and the filteredsignal.
 44. The system of claim 43, wherein each of the power detectorsincludes a sampler configured to take samples of its respective signalwithin windows thereof.
 45. The system of claim 43 wherein each of thepower detectors does not use highest magnitude samples in themeasurement of its respective signal.
 46. The system of claim 39,wherein the filtered signal power measurement uses at least certainsignal samples from the received signal power measurement.
 47. Thesystem of claim 39 further including a DC offset control unit forremoving DC offset from the input signal.
 48. The system of claim 39wherein the control logic includes: means for determining whether thepower of the received digital signal is greater than a desired level andif so identifying the received digital signal as the strong signal type;49. The system of claim 48 further including: means for reducing a gainprovided by an automatic gain control circuit to an automatic gaincontrol amplification section if the power of the received digitalsignal is greater than the desired level.
 50. The system of claim 49wherein the means for reducing the gain reduces the gain by an amountthat is based upon the determined power and a predetermined gain targetvalue if the power of the received digital signal is greater than thedesired level.
 51. The system of claim 49 further including: means fordetermining whether the received digital signal saturates the analog todigital converter and if so identifying the digital signal as the strongsignal type.
 52. The system of claim 51 wherein the means fordetermining includes means for counting a number of times that aplurality of samples of the received digital signal exceeded apredetermined saturation threshold; and means for comparing the numberof times with a predetermined saturation count threshold, such that whenthe predetermined saturation threshold is exceeded, the received digitalsignal is determined to saturate the analog to digital converter
 53. Thesystem of claim 51 further including: means for reducing the gainprovided by the automatic gain control circuit to the automatic gaincontrol amplification section if the received digital signal isdetermined to saturate the analog to digital converter.
 54. The methodof claim 36 wherein the means for reducing the gain reduces the gain apredetermined amount if the received digital signal is determined tosaturate the analog to digital converter.
 55. A method of determiningthat a received digital signal of the strong signal type exists withinan input signal including the steps of: converting the input signal to areceived digital signal using an analog to digital converter;determining whether the received digital signal saturates the analog todigital converter; and reducing a gain provided by an automatic gaincontrol circuit to an automatic gain control amplification section ifthe received digital signal is determined to saturate the analog todigital converter.
 56. The method of claim 55 wherein the step ofreducing the gain reduces the gain a predetermined amount.
 57. Themethod of claim 55 wherein the step of determining whether the receiveddigital signal saturates the analog to digital converter includes thesteps of: counting a number of times that a plurality of samples of thereceived digital signal exceeded the predetermined saturation threshold;and comparing the number of times with a predetermined saturation countthreshold, such that when the predetermined saturation threshold isexceeded, the received digital signal is determined to saturate theanalog to digital converter.